Many musical instruments utilize selective opening and closure of numerous openings in the instrument to change the sound produced by the instrument. Flutes, for example, typically have seventeen or eighteen pads which cover respective tone holes. Accurate and repeatable sealing of the tone holes is essential for producing crisp, clean tones. Other instruments utilize pads to selectively cover "spit" holes through which condensed moisture is expelled.
The most prevalent pads are a composite of cardboard, wool felt, and a collagen membrane covering the felt and attached to the cardboard. The pad is secured within a pad cup by a screw and washer or an adhesive. However, a great deal of effort is required to properly align the pad with the rim of the opening over which the pad is positioned. Highly skilled padders utilize paper or plastic shims of about 0.001 to 0.012 inch thick to adjust the alignment of the pad. Alternatively, certain pads can be floated into position on a bed of shellac or other adhesive. Each shimming adjustment requires the removal of the pad from the pad cup to position new shims. Proper padding of the flute often takes three or more days to accomplish. Even with proper shimming or floating, the pads are limited in their ability to conform to irregularities in the rim.
Moreover, the collage; membranes, referred to as "fish skins", are subject to charge over time. The membranes have different fiber densities which differentially absorb moisture resulting in differential swelling. Differential swelling of the pad defeats a proper seal between the rim and the pad, and can also result in excessive wear in portions of the membrane covering.
Great effort has been expended in developing new pads which attempt to address one or more of the above-mentioned problems. Some of these pads still require complicated shimming, such as by selectively bending a metal collar. Others rely on synthetic materials to deform sufficiently to establish a proper seal. Many of these pads are too soft, however, and produce a "spongy" feel during use. Instead, a crisp, slightly resilient action is desired.
Pad materials must withstand a harsh environment involving corrosive saliva and other moisture, and rapid changes in temperature. Further, it is important for the pad to be quiet during operation and not to adhere to the rim. It is also desirable to minimize buildup of static charge, to resist mildew, and to combat odors which can develop when the instrument is left in its carrying case.